The present invention relates to a process for producing pure water, and more particularly, it relates to an economical process for producing pure water comprising using a specific anion exchange resin.
In the art of pure water production in recent years, there is a tendency of incorporating a desalting step in the process for the sake of quality control. In fact, the process using a high purity water system of a double bed system or mixed bed system using a strongly acidic cation exchange resin and a strongly basic anion exchange resin is widely adopted in practice as such process is capable of removing substantially all of the ions dissolved in water.
Various types of strongly acidic cation exchange resins and strongly basic anion exchange resins have been used in the art. For instance, as disclosed in Japanese Patent Application Laid-open (Kokai) Nos. 4-349941 and 7-289921, a strongly basic anion exchange resins having the structural units represented by the following formula (I) are known as heat resistant resins: ##STR1## wherein A is a C.sub.3 -C.sub.6 straight-chain alkylene group or a C.sub.5 -C.sub.7 alkoxymethylene group; R is a C.sub.1 -C.sub.4 alkyl group which may be substituted with a hydroxyl group; and X is a counter ion coordinated to an ammonium group.
Also, in Japanese Patent Application Laid-open (Kokai) Nos. 5-49949, 5-49950, 5-49951 and 7-289920 are disclosed a process for producing pure water, a process for producing water for pharmaceuticals or cosmetics, and a method for decolorization of saccharic solution using strongly basic anion exchange resins. In any of these methods, however, attention is focused only on heat resistance of the strongly basic anion exchange resins. Especially, Japanese Patent Application Laid-open (Kokai) No. 5-49950 teaches that desalted water with no remnant of amine smell can be produced by using the strongly basic anion exchange resins, but this Japanese Kokai is silent on such matters as concentrations of the eluted substances in the produced desalted water and the concrete production conditions.
Most of the high purity water systems using conventional strongly basic anion exchange resins or known anion exchange resins (Japanese Patent Application Laid-open (Kokai) No. 5-49950) are at a disadvantage in that they are incapable of producing tasteless and odorless pure water soon after start of the operation of the high purity water system. The substances eluted from the strongly basic anion exchange resin, such as amines and acetaldehydes, are held responsible for the lingering taste and odor in the produced pure water. Therefore, in order to produce pure water with a trimethylamine concentration of not more than 20 ppb and a total organic carbon content of not more than 200 ppb, which are the standard requirements for the tasteless and odorless potable water, it is necessary to previously pass ultrapure water of an amount of several thousand times the volume of the strongly basic anion exchange resin through the high purity water system. This is a serious economical disadvantage. Further, it is impossible to produce pure water instantly after start of the operation of the high purity water system in case of passing at a space velocity of not less than 20.
As a result of the present inventors' extensive researches, it has been found that by using as an ion exchange resin a specific heat-resistant strongly basic anion exchange resin, and passing raw water at a space velocity of not less than 20, it is possible to produce pure water having a total organic carbon content of not more than 200 ppb instantly after start of the operation of a high purity water system. The present invention has been attained on the basis of the above finding.